Magnetic data recording media in the form of rotating disks are extensively used as memories which allow rapid, random access to information stored on the disk. There is a continuing effort to increase the amount of information stored on a disk by increasing the density which is commonly referred to as areal density or bits per square inch. The density is dependent upon the properties of the magnetic particles and the thickness of the coating. In general, the thinner the coating, the higher the recording density can be. When higher densities and thinner coatings are used, the heads, which fly over the disk surface on a layer or film of air must brought closer to the coated surface of the disk. As the distance between the flying head and the disk surface is reduced, for example, from 12 to 18 microinches to a height of 1 to 6 microinches, it will be appreciated that the flatness of the recording surface must be even further improved.
A recognized technique for the manufacture of disk memories is the coating of an aluminum disk substrate with a liquid dispersion which contains a magnetizable material finely dispersed in a curable binder, subsequently drying and/or curing this coating and, if desired, also buffing and/or polishing it. A successful coating technique is the spin coating process, in which the disk substrate is caused to rotate and the coating mixture is allowed to flow onto it, as described for example in U.S. Pat. No. 3,198,657. To achieve a thin magnetic layer, the excess dispersion which has been applied is then spun off.
Subsequently it was found that, by placing an airflow barrier over a magnetic oxide coated disk during the spin off portion of the spin coat cycle, coating thickness could be reduced and other properties improved. This process requires placing a plate referred to as an air barrier over the disk surface during the spin-off process step. The air barrier reduces air circulation, thus permitting a solvent rich environment to form over the surface of the disk. This causes additional solvent evaporation to be slowed and as a result, the viscosity of the ink does not increase as rapidly during spin-off. With a lower viscosity, the coating will flow thinner and more uniformly resulting in a reduction in the as-coated thickness.
One air barrier approach is the use of a barrier that is corotating with the disk. This mode of operation is shown in U.S. Pat. No. 4,201,149 for the purpose of eliminating coating streaks that appear in a manner resembling the spokes of a wheel and usually occur when thin layer coatings are applied. The plate is positioned as mush as 20 mm above the coated disk and the disk and barrier are spun together producing the aerodynamic boundary layer that achieves the effect provided by the teaching. Another technique is to place a static air barrier plate over the disk surface during the spin-off step. This process is started by traversing a coating nozzle across a spinning disk during which the magnetic ink is applied. Once the nozzle has cleared the disk, the air barrier assembly is lowered over the disk surface in close proximity, often as close as 18 thousandths of an inch. This is immediately followed by a spin-off process step, orientation and a spin dry process. By having the air barrier over the disk during spin-off, a marked reduction in the as-coated thickness can be obtained. An example of this technique is U.S. patent application Ser. No. 500,067 filed June 1, 1983 now U.S. Pat. No. 4,485,755 (Coating Thickness and Wedge Geometry Control for Magnetic Disks).
An adverse phenomenon that may occur during the spin-off cycle is spoking, wherein streaks in the coating are formed that resemble the spokes of a wagon wheel. These coating streaks increase the roughness of the disk surface resulting in coating thickness variations that can not be buffed out.